The present invention relates to a diversity receiver system and, in particular, relates to an antenna selection diversity receiver system for overcoming fading in radio communication system.
In a radio communication system, receive level is subject to change because of fading, and the decrease of receive level causes the deterioration of signal quality. In order to solve that problem and to provide stable and high quality information transmission, a number of diversity techniques have been proposed. In particular, an antenna selection diversity reception system is considered to be promising for a mobile telephone system in which a miniaturized receiver set with low power consumption is essential.
FIG. 10 shows a prior antenna selection diversity receiver for a 3 channel TDM system (time division multiplex). That kind of prior art is shown in IEEE Global Communication Conference 1988, entitled "Performance Of a Novel Selection Diversity Technique In an Experimental TDMA System For Digital Portable Radio Communications", pages 26.2.1-26.2.5. In the figure, the numerals 1 and 2 are receive antennas, 3 is a selection switch, 4 is a receiver/demodulator, 5 is a frame synchronization circuit, 6 is an antenna selection circuit, 6-1 is a timing circuit, 6-2 is a switch control circuit, 6-3 is a receive level measurement circuit, 6-4 and 6-5 are sample-hold circuits, and 6-6 is a comparator.
The signal A1 in the antenna 1 and the signal A2 in the antenna 2 are switched by the selection switch 3, and the selected signal A1 or A2 is applied to the receiver/demodulator 4, which demodulates the receive signal. The demodulated signal is applied to the frame synchronization circuit 5 for the frame synchronization. The antenna selection circuit 6 receives the frame timing signal from the frame synchronization circuit 5, and recognizes the receive timing of an assigned burst slot to its own receive station. The selection switch 3 is switched just before that beginning of the signal burst slot of its own station so that the receive levels on two antennas are compared.
FIG. 11 shows a switching process in the prior art, in which (a) shows a frame structure of a receive signal, which is a TDM signal with three channels in each frame. In this embodiment, the third burst in each frame is the time slot assigned to the own station. The timing circuit 6-1 forwards to the receive level measurement circuit 6-3 a trigger signal at time t.sub.1 which is just before the beginning of the assigned time slot so that the receive level R.sub.1 at the antenna 1 is kept in the sample-hold circuit 6-4. Then, the switch control circuit 6-2 switches the selection switch 3 to the other antenna 2 so that the receive level R.sub.2 at the antenna 2 at time t.sub.2 is kept in the sample-hold circuit 6-5. The comparator 6-6 compares R.sub.1 with R.sub.2. The switch control circuit 6-2 selects the antenna which provides the higher receive level according to the result of the comparison, and keeps the switch 3 to receive the assigned time slot according to said comparison result.
For instance, assuming that the receive level in the antennas 1 and 2 changes as shown by the curves C1 and C2 in FIG. 11(b), the switch control circuit 6-2 forwards the selection signal as shown in FIG. 11(c), in which the curve (1) indicates the selection of the antenna 1, and the curve (2) shows the selection of the antenna 2. R.sub.1 is the level on the curve C1 just before the assigned time slot when the antenna 1 is selected, and R.sub.2 is the receive level on the curve C2 just before the assigned time slot when the antenna 2 is selected. As the level relations R.sub.1 &gt;R.sub.2 is satisfied at time t.sub.1 (or t.sub.2) just before the assigned time slot, the antenna 1 is selected for receiving the assigned burst signal. The receive level is compared before each assigned slot, and an antenna which provides the higher receive level is selected so that the receive level applied to the receiver/demodulator 4 is kept high to prevent data error.
However, the prior art has the disadvantage that little diversity effect is obtained when fading rate is high, or the receive level changes rapidly. When the receive level changes after the levels R.sub.1 and R.sub.2 are measured at time t.sub.1 or t.sub.2, the selected antenna does not provide the higher receive level.
FIG. 11(d) and FIG. 11(e) show the above situation. When the receive levels of the antennas 1 and 2 change as shown in the curves C1 and C2 in FIG. 11(d), the relations R.sub.1 &gt;R.sub.2 are satisfied at time t.sub.1 and t.sub.2, and therefore, the antenna 1 is selected for receiving the assigned burst signal. However, after the antenna is switched, the receive levels are reversed, and the selected antenna provides the lower receive signal. In that case, an error rate is not improved by an antenna selection diversity system.
As mentioned above, the prior system is not useful when fading rate is high.